Tunneling magnetoresistance (TMR) device, its manufacture method, magnetic head and magnetic memory using TMR device

ABSTRACT

A barrier layer is disposed over a pinned layer made of ferromagnetic material having a fixed magnetization direction, the barrier layer having a thickness allowing electrons to transmit therethrough by a tunneling phenomenon. A first free layer is disposed over the barrier layer, the first free layer being made of amorphous or fine crystalline soft magnetic material which changes a magnetization direction under an external magnetic field. A second free layer is disposed over the first free layer, the second free layer being made of crystalline soft magnetic material which changes a magnetization direction under an external magnetic field and being exchange-coupled to the first free layer. A tunneling magnetoresistance device is provided which has good magnetic characteristics and can suppress a tunnel resistance change rate from being lowered.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and claims priority of Japanese PatentApplication No. 2006-307987 filed on Nov. 14, 2006, the entire contentsof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

A) Field of the Invention

The present invention relates to a tunneling magnetoresistance deviceand its manufacture method, and more particularly to a tunnelingmagnetoresistance device which changes its electric resistance dependingon an external magnetic field and is applied to a reproducing head of amagnetic recording apparatus and a magnetic memory, and its manufacturemethod.

B) Description of the Related Art

In a junction having a “metal/insulating film/metal” structureconsisting of the insulating film and metal films sandwiching theinsulating film therebetween, as a voltage is applied across opposingmetal layers, a small current flows if the insulating film issufficiently thin. Generally, current does not flow through aninsulating film. However, if the insulating film is sufficiently thin,e.g., several nm or thinner, electrons transmit through the insulatingfilm at some probability because of the quantum mechanics effects.Current of electrons transmitting through an insulating film is called a“tunnel current” and its structure is called a “tunnel junction”.

Generally, a metal oxide film is used as the insulating film of thetunnel junction. For example, a thin insulating film of aluminum oxideis formed by natural oxidation, plasma oxidation or thermal oxidation ofa surface layer of aluminum. By controlling oxidation conditions, aninsulating film can be formed which is applicable to the tunnel junctionand has a thickness of several nm.

A device having a tunnel junction exhibits nonlinear current-voltagecharacteristics and has been used as a nonlinear device.

The structure of the tunnel junction whose opposing metal layers aremade of ferromagnetic material is called a “ferromagnetic tunneljunction”. A tunnel probability (tunnel resistance) of a ferromagnetictunnel junction is dependent upon a magnetization state of opposingferromagnetic materials. Therefore, the tunnel resistance can be changedby controlling the magnetization state by applying an external magneticfield. A tunnel resistance R can be expressed by the following equation:

R=Rs+0.5ΔR(1−cosθ)

where θ is a relative angle between magnetization directions of opposingferromagnetic materials. Rs represents a tunnel resistance at themagnetization direction relative angle θ of 0, i.e., at parallelmagnetization directions, and ΔR represents a difference between tunnelresistances at the magnetization direction relative angle θ of 180°,i.e., at counter-parallel magnetization directions and the tunnelresistance at the parallel magnetization directions.

A phenomenon that a tunnel resistance changes depending on amagnetization direction of ferromagnetic material results frompolarization of electrons in ferromagnetic material. Generally, thereexist in metal, spin-up electrons in an upward spin state and spin-downelectrons in a downward spin state. There exist in nonmagnetic metal,the same number of spin-up electrons and spin-down electrons. Therefore,no magnetism is exhibited as a whole. In ferromagnetic material, thenumber of spin-up electrons (Nup) is different from the number ofspin-down electrons (Ndown) so that the ferromagnetic material exhibitsspin-up or spin-down magnetism as a whole.

It is known that when an electron transmits through a barrier layer bythe tunnel phenomenon, the spin state of the electron is retained.Therefore, if there is a vacant electron quantum level in a tunneldestination ferroelectric material, an electron can transmit through thebarrier layer. If there is no vacant electron quantum level, an electroncannot transmit through the barrier layer.

A change rate ΔR/Rs of a tunnel resistance is expressed by the followingequation:

ΔR/Rs=2P ₁ P ₂/(1−P ₁ P ₂)

wherein P₁ and P₂ are spin polarizabilities of ferroelectric material onboth sides of a barrier layer. The spin polarizability is given by thefollowing equation:

P=2(Nup−Ndown)/(Nup+Ndown)

Tunneling magnetoresistance devices are reported in “Japanese PatentPublication No. 2871670”, “Yuasa et al., Nature Materials vol. 3 (2004)p. 868-p. 871”, “Parkin et al., Nature Materials vol. 3 (2004) p. 862-p.867”, and “Tsunekawa et al., Effect of Capping Layer Material on TunnelMagnetoresistance in CoFeB/MgO/CoFeB magnetic Tunnel Junctions,International Magnetic Conference 2005, HP-08, p. 992”.

SUMMARY OF THE INVENTION

Magnetic characteristics of a tunneling magnetoresistance device areimproved. A tunnel resistance change rate of a tunnelingmagnetoresistance device can be suppressed from being lowered. Further,a manufacture method for such a tunneling magnetoresistance device isprovided.

According to one aspect of the present invention, there is provided atunneling magnetoresistance device including:

a pinned layer made of ferromagnetic material having a fixedmagnetization direction;

a barrier layer disposed over the pinned layer and having a thicknessallowing electrons to transmit therethrough by a tunneling phenomenon;

a first free layer disposed over the barrier layer and made of amorphousor fine crystalline soft magnetic material which changes a magnetizationdirection under an external magnetic field; and

a second free layer disposed over the first free layer and made ofcrystalline soft magnetic material which changes a magnetizationdirection under an external magnetic field and being exchange-coupled tothe first free layer.

According to another aspect of the present invention, there is provideda method for manufacturing a tunneling magnetoresistance device,including steps of:

(a) forming a pinning layer made of antiferromagnetic material on asupport substrate;

(b) forming a pinned layer over the pinning layer, the pinned layerbeing made of ferromagnetic material whose magnetization direction isfixed by an exchange interaction with the pinning layer;

(c) forming a barrier layer over the pinned layer, the barrier layerhaving a thickness allowing electrons to transmit therethrough by atunneling phenomenon;

(d) forming a first free layer made of amorphous or fine crystallinesoft magnetic material over the barrier layer;

(e) exposing a surface of the first free layer to nitrogen plasma;

(f) forming a second free layer made of crystalline soft magneticmaterial over the first free layer exposed to the nitrogen plasma; and

(g) conducting a regularizing heat treatment process for the pinninglayer by disposing a lamination structural body between the supportsubstrate and the second free layer in a magnetic field.

According to still another aspect of the present invention, there isprovided a method for manufacturing a tunneling magnetoresistancedevice, including steps of:

(a) forming a pinning layer made of antiferromagnetic material on asupport substrate;

(b) forming a pinned layer over the pinning layer, the pinned layerbeing made of ferromagnetic material whose magnetization direction isfixed by an exchange interaction with the pinning layer;

(c) forming a barrier layer over the pinned layer, the barrier layerhaving a thickness allowing electrons to transmit therethrough by atunneling phenomenon;

(d) forming a first free layer made of amorphous or fine crystallinesoft magnetic material over the barrier layer;

(e) forming a crystallization suppressing layer over the first freelayer;

(f) forming a second free layer made of crystalline soft magneticmaterial over the crystallization suppressing layer; and

(g) conducting a regularizing heat treatment process for the pinninglayer by disposing a lamination structural body between the supportsubstrate and the second free layer in a magnetic field,

wherein the crystallization suppressing layer suppressing the first freelayer from being crystallized by inheriting a crystal structure of thesecond free layer during the step (g).

According to still another aspect of the present invention, there isprovided a magnetic head provided with the tunneling magnetoresistancedevice.

According to still another aspect of the present invention, there isprovided a magnetic memory including:

the tunneling magnetoresistance device;

recording means for applying a magnetic field to the tunnelingmagnetoresistance device to change magnetization directions of first andsecond free layers of the tunneling magnetoresistance device; and

reproducing means for applying a sense current through the tunnelingmagnetoresistance device to detect a resistance of the tunnelingmagnetoresistance device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a cross sectional view and a plan view of atunneling magnetoresistance device according to a first embodiment.

FIGS. 2A to 2D are cross sectional views of the tunnelingmagnetoresistance device during manufacture of the first embodiment.

FIGS. 3A and 3B are graphs showing the relation between a resistancechange rate and an applied magnetic field of tunneling magnetoresistancedevices of the first embodiment and a comparative example, respectively.

FIGS. 4A and 4B are TEM photographs in section of the tunnelingmagnetoresistance devices of the first embodiment and the comparativeexample, respectively.

FIG. 5 is a cross sectional view of a tunneling magnetoresistance deviceaccording to a second embodiment.

FIGS. 6A and 6B are graphs showing the relation between a resistancechange rate and an applied magnetic field of tunneling magnetoresistancedevices of the second embodiment and a comparative example,respectively.

FIG. 7 is a front view of a magnetic head using the tunnelingmagnetoresistance device of each of the first and second embodiments.

FIG. 8A is a cross sectional view of an MRAM using the tunnelingmagnetoresistance device of each of the first and second embodiments,and FIG. 8B is an equivalent circuit of MRAM.

FIG. 9A is a cross sectional view of a tunneling magnetoresistancedevice consisting of a CoFeB/MgO/CoFeB lamination structure, and FIG. 9Bis a graph showing the relation between a resistance change rate and anapplied magnetic field.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A tunneling magnetoresistance device consisting of a CoFeB/MgO/CoFeBlamination structure is explained below prior to describing embodiments.

FIG. 9A shows an example of a tunneling magnetoresistance deviceconsisting of a CoFeB/MgO/CoFeB lamination structure. An underlyinglayer 101 of Ta having a thickness of 50 nm, a pinning layer 102 of PtMnhaving a thickness of 15 nm, a first pinned layer 103 of CoFe having athickness of 3 nm, a non-magnetic coupling layer 104 of Ru having athickness of 0.8 nm, a second pinned layer 105 of CoFeB having athickness of 3 nm, a barrier layer 106 of MgO having a thickness of 2nm, a free layer 107 of CoFeB having a thickness of 3 nm, a first caplayer 108 of Ta having a thickness of 10 nm and a second cap layer 109of Ru having a thickness of 10 nm are formed in this order on a supportsubstrate 100 made of Si or SiO₂.

FIG. 9B shows a relation between an external magnetic field and aresistance change rate. A resistance change rate is defined by (R−Rs)/Rswhere Rs is a device resistance when a magnetization direction of thepinned layers 103 and 105 is parallel to a magnetization direction ofthe free layer 107, and R is a device resistance when an externalmagnetic field is applied. It can be seen that the maximum resistancechange rate of about 200% is obtained.

When a tunneling magnetoresistance device is applied to a magnetic head,it is required to have desired magnetic characteristics, that ismagnetization characteristics, magnetostriction characteristics,coercive force, magnetic anisototropy and the like. For example, it canbe known from the measurement results shown in FIG. 9B that a magneticfield (coercive force) of about 50 Os is required to reversemagnetization of the free layer 107 of the tunneling magnetoresistancedevice. In order to apply the device to a magnetic head, it is necessaryto lower the coercive force. An effective coercive force can be loweredby stacking soft magnetic material having a smaller coercive force thanCoFeB on the free layer 107 made of CoFeB.

If a layer of soft magnetic material having a small coercive force suchas NiFe is stacked on the free layer of CoFeB, a resistance change ratelowers.

A first embodiment is described below.

FIGS. 1A and 1B are a cross sectional view and a plan view of atunneling magnetoresistance device according to the first embodiment.FIG. 1A corresponds to a cross sectional view taken along one-dot chainline 1A-1A of FIG. 1B.

As shown in FIG. 1A, a conductive layer 12 of NiFe is formed on asupport substrate 10 having an SiO₂ film formed on Si. Other materialssuch as ceramic material such as AlTiC, and quartz glass may be used asthe material of the support substrate 10. The surface of the NiFeconductive layer 12 is planarized by chemical mechanical polishing(CMP). A tunneling magnetoresistance device 40 of a cylindrical shape isformed on a partial area of the conductive layer 12.

The tunneling magnetoresistance device 40 is formed by laminating afirst underlying layer 13, a second underlying layer 14, a pinning layer18, a first pinned layer 20, a non-magnetic coupling layer 21, a secondpinned layer 22, a barrier layer 25, a first free layer 30, a secondfree layer 32, a first cap layer 35 and a second cap layer 36 in thisorder.

The first underlying layer 13 is made of Ta and has a thickness of about5 nm. The first underlying layer 13 may be made of Cu or Au, or may be alamination layer of these materials. The second underlying layer 14 ismade of Ru and has a thickness of about 2 nm.

The pinning layer 18 is made of IrMn and has a thickness of about 7 nm.The pinning layer 18 may be made of antiferromagnetic material otherthan IrMn, such as alloy of Mn and at least one element selected from agroup consisting of Pt, Pd, Ni, Ir and Rh. A thickness of the pinninglayer 18 is preferably in a range between 5 nm and 30 nm, and morepreferably in a range between 10 nm and 20 nm. The pinning layer 18 isregularized by heat treatment in a magnetic field after it is deposited,and exhibits antiferromagnetism.

The first pinned layer 20 is made of Co₇₄Fe₂₆ and has a thickness of,e.g., 2 nm. The non-magnetic coupling layer 21 is made of Ru and has athickness of, e.g., 0.8 nm. The second pinned layer 22 is made ofCO₆₀Fe₂₀B₂₀ and has a thickness of, e.g., 2 nm. A magnetizationdirection of the first pinned layer 20 is fixed to a certain directionby an exchange interaction with the pinning layer 18. Namely, themagnetization direction of the first pinned layer 20 does not changeeven if an external magnetic field is applied if the magnetic fieldintensity is weaker than the exchange interaction. The first pinnedlayer 20 and second pinned layer 22 exchange-coupleantiferromagnetically with each other via the non-magnetic couplinglayer 21.

A thickness of the non-magnetic coupling layer 21 is set in a rangeallowing that the first pinned layer 20 and second pinned layer 22exchange-couple antiferromagnetically with each other. The thickness isin a range between 0.4 nm and 1.5 nm, and preferably between 0.4 nm and0.9 nm. The first pinned layer 20 and second pinned layer 22 may be madeof ferromagnetic material which contains one of Co, Ni and Fe. Thenon-magnetic coupling layer 21 may be made of non-magnetic material suchas Rh, Ir, Ru-based alloy, Rh-based alloy and Ir-based alloy, inaddition to Ru. Alloy containing Ru and at least one element selectedfrom a group consisting of Co, Cr, Fe, Ni and Mn may be cited as anexample of the Ru-based alloy.

The magnetization direction of the first pinned layer 20 and themagnetization direction of the second pinned layer 22 arecounter-parallel so that an intensity of a net leakage magnetic fieldfrom the first and second pinned layers 20 and 22 lowers. This mitigatesthe adverse effect that the leakage magnetic field changes themagnetization directions of the first and second free layers 30 and 32.Accordingly, magnetization of the first and second free layers 30 and 32can respond correctly to a leakage magnetic field from a magneticrecording medium, and detection accuracy for magnetization recorded inthe magnetic recording medium is improved.

The barrier layer 25 is made of MgO and has a thickness of, e.g., 1.0nm. It is preferable that MgO of the barrier layer 25 is crystalline,and it is particularly preferable that the (001) plane of MgO isoriented generally in parallel to the substrate surface. A thickness ofthe barrier layer 25 is preferably in a range between 0.7 nm and 2.0 nmfrom the viewpoint of good film quality. The barrier layer 25 may bemade of AlO_(x), TiO_(x), ZrO_(x), AlN, TiN, ZrN or the like, in placeof MgO. If the barrier layer 25 is made of material other than MgO, itsthickness is preferably in a range between 0.5 nm and 2.0 nm, and morepreferably in a range between 0.7 nm and 1.2 nm.

The first free layer 30 is made of amorphous CO₆₀Fe₂₀B₂₀ and has athickness of about 2 nm. From the viewpoint that the first free layer 30is easy to be amorphous, a B concentration is preferably in a rangebetween 10 atom % and 25 atom %. The first free layer 30 may be made ofsoft magnetic material added with at least one element selected from agroup consisting of B, C, Al, Si and Zr, in place of CoFeB.

The second free layer 32 is made of Ni₈₀Fe₂₀ and has a thickness of,e.g., 4 nm. The second free layer 32 is made of soft magnetic materialhaving a smaller coercive force than that of the first free layer 30.CoNiFe having a composition allowing a face centered cubic structure maybe cited as an example of the material of the second free layer 32, inplace of NiFe. At least one element selected from a group consisting ofB, C, Al, Si and Zr may be added to NiFe and CoNiFe. A concentration ofthe added element is set lower than that of the element added to thefirst free layer 30.

By ferromagnetically coupling the second free layer 32 having a smallercoercive force to the first free layer 30, sensitivity to a change inthe external magnetic field can be improved. Generally, a ferromagneticfilm is more sensitive to a change in the direction of an externalmagnetic field, the smaller the coercive force is. Since the coerciveforce of the second free layer 32 is lower than that of the first freelayer 30, as the direction of an external magnetic field changes, themagnetization direction of the second free layer 32 changes before themagnetization direction of the first free layer 30 changes. Since thefirst free layer 30 is ferromagnetically exchange-coupled to the secondfree layer 32, the magnetization direction of the first free layer 30changes following a change in the magnetization direction of the secondfree layer 32. Therefore, the magnetization direction of the first freelayer 30 is more susceptible to a change in the direction of an externalmagnetic field. Since the magnetization direction of the first freelayer 30 contributes to the resistance change rate, a sensitivity of thetunneling magnetoresistance device can be improved by disposing thesecond free layer 32.

The first cap layer 35 is made of Ta and has a thickness of, e.g., 5 nm.The second cap layer 36 is made of Ru and has a thickness of, e.g., 10nm. The first cap layer 35 and second cap layer 36 prevent theunderlying ferromagnetic layer and the like from being oxidized duringheat treatment. The first cap layer 35 may be made of Ru, and the secondcap layer 36 may be made of Ta. More generally, the cap layer may bemade of non-magnetic metal such as Au, Ta, Al, W and Ru, or may be madeof a lamination structure of layers made of these metals. A totalthickness of the cap layers is preferably in a range between 5 nm and 30nm.

Of the surface of the conductive layer 12, the region where thetunneling magnetoresistance device 40 is not disposed is covered with aninsulating film 48 of insulating material such as SiO₂. A firstelectrode 45 is formed on the tunneling magnetoresistance device 40 andinsulating film 48. The first electrode 45 is electrically connected tothe second cap layer 36. A via hole is formed through the insulatingfilm 48, reaching the conductive layer 12. The via hole is filled with asecond electrode 46. The second electrode 46 is electrically connectedto the conductive layer 12. The first electrode 45 and second electrode46 are made of, e.g., Cu.

Next, with reference to FIGS. 2A to 2D, description will be made on amanufacture method for the tunneling magnetoresistance device of thefirst embodiment.

As shown in FIG. 2A, the layers from the conductive layer 12 to firstfree layer 30 are formed on the support substrate 10 by using amagnetron sputtering system.

As shown in FIG. 2B, the first free layer 30 is exposed to nitrogenplasma 38. For example, this plasma process is executed under thefollowing conditions:

-   -   Nitrogen gas flow rate: 100 sccm    -   RF power: 50 W    -   Process time: 30 sec

As shown in FIG. 2C, the second free layer 32, first cap layer 35 andsecond cap layer 36 are formed on the first free layer 30 subjected tothe surface treatment by nitrogen plasma, by using the magnetronsputtering system. Thereafter, the substrate is disposed in vacuum and aregularizing heat treatment process is performed for the pinning layer18, in the state that a magnetic field is applied. A heat treatmenttemperature is, e.g., 270° C. and a heat treatment time is, e.g., fourhours. The heat treatment temperature may be in a range between 250° C.and 400° C.

As shown in FIG. 2D, the layers between the first underlying layer 13and the second cap layer 36 are patterned to form the tunnelingmagnetoresistance device 40 of a cylindrical shape. Patterning theselayers may be performed by Ar ion milling. Thereafter, as shown in FIG.1A, the insulating film 48, first electrode 45, via hole through theinsulating film 48 and second electrode 46 are formed.

FIG. 3A shows a resistance change rate of a tunneling magnetoresistancedevice manufactured by the method of the first embodiment. For thereference sake, a resistance change rate is also shown for a comparativeexample manufactured without the nitrogen plasma process shown in FIG.2B. The tunneling magnetoresistance device of the comparative examplewithout the plasma nitrogen process has the maximum resistance changerate of about 20%, whereas the tunneling magnetoresistance devicemanufactured by the first embodiment method has the maximum resistancechange rate of about 60%. Since the second free layer 32 having asmaller coercive force is disposed on the first free layer 30, aneffective coercive force of the free layer was 50 Os or weaker. Theadvantage of disposing the second free layer 32 is apparent as comparedto the coercive force of about 500 Os of the free layer 107 of thetunneling magnetoresistance device shown in FIG. 9A.

The coercive force can be made small by disposing the second free layer32. However, if the second free layer 32 is disposed simply on the firstfree layer 30, the resistance change rate lowers as shown in FIG. 3B. Asin the first embodiment, the resistance change rate can be maintainedhigh by exposing the first free layer 30 to the nitrogen plasma 38 afterthe first free layer 30 is formed and before the second free layer 32 isformed. In the following, studies will be made on the reason why theresistance change rate can be maintained high.

FIGS. 4A and 4B show transmission electron microscope (TEM) photographsof the cross sections of tunneling magnetoresistance devices of thefirst embodiment and a comparative example, respectively. In thecomparative example, it can be seen from FIG. 4B that the first freelayer 30 of CoFeB is polycrystallized. It can be considered thatcrystallization progresses from the interface between the first freelayer 30 and second free layer 32 toward the inside of the first freelayer 30 during the regularizing heat treatment for the pinning layer 18and other heat treatment. Therefore, although the first free layer 30 isamorphous immediately after film formation, the first free layer 30 iscrystallized by the succeeding heat treatment. It can be understood froma distance between crystallographic planes in the TEM photograph thatCoFeB of the first free layer 30 has the (111) plane preferentiallyoriented in parallel to the substrate surface.

As shown in FIG. 4A, in the first embodiment, a crystalline structure isnot observed in the first free layer 30, and the first free layer 30 isamorphous. It is known that if the ferromagnetic layer being in contactwith the barrier layer 25 has the (111) orientation, the resistancechange rate lowers. In the first embodiment, the resistance change rateis suppressed from lowering, by making the first free layer 30amorphous.

It can be seen from the TEM photograph that NiFe of the second freelayer 32 of the tunneling magnetoresistance device of the firstembodiment has the (111) orientation. Since the second free layer 32 isnot in contact with the barrier layer 25, the (111) orientation of thesecond free layer 32 does not cause a lowered resistance change rate.

In the first embodiment, the second free layer 32 is made of crystallineferromagnetic material having the face centered cubic structure and the(111) orientation. After the first free layer 30 is formed, the surfacethereof is subjected to the plasma process. It is therefore possible toprevent the first free layer 30 from being crystallized by inheritingthe crystal structure of the second free layer 32 on the first freelayer 30. Even if the orientation of crystalline grains of the secondfree layer 32 is random (namely the second free layer 32 hasnon-orientation.), it is possible to suppress the resistance change ratefrom being lowered, by making the first free layer 30 amorphous.

In the first embodiment, a composition ratio of Co, Fe and Bconstituting the first free layer 30 is set to 60 atom %, 20 atom % and20 atom %, respectively. B is added in order to make CoFe alloyamorphous. In order to make the first free layer 30 amorphous, it ispreferable to set a B concentration to 10 atom % or higher.

Generally, it is difficult to definitely distinguish amorphous statefrom fine crystalline state. As shown in FIG. 4B, if clear crystallattice images can be observed in the first free layer 30, it can bedefined that the first free layer 30 is crystalline. If clear crystallattice images cannot be observed, it can be defined that the first freelayer 30 is either amorphous or fine crystalline. Even if the first freelayer 30 is fine crystalline, it is possible to suppress the resistancechange rate from being lowered more than if the first free layer 30 iscrystalline. If a sharp peak does not appear in an X-ray diffractionpattern of CoFeB constituting the first free layer 30, it can be definedthat the first free layer 30 is either amorphous or fine crystalline.

A very thin region near the interface between the barrier layer 25 andfirst free layer 30 is crystallized in some cases. However, if most ofthe region of the first free layer 30 are amorphous or fine crystalline,it is possible to obtain sufficient advantages of suppressing theresistance change rate from being lowered. If a very thin crystallizedregion has a thickness of at most 0.5 nm, it can be defined that thefirst free layer 30 is amorphous or fine crystalline as a whole.

FIG. 5 is a cross sectional view of a tunneling magnetoresistance deviceaccording to the second embodiment. In the second embodiment, acrystallization suppressing layer 50 is inserted between the first feelayer 30 and second free layer 32. The crystallization suppressing layer50 is a Ta layer having a thickness of, e.g., 0.2 nm, and is formed bymagnetron sputtering. In the second embodiment, the surface of the firstfree layer 30 is not processed by nitrogen plasma shown in FIG. 2B ofthe first embodiment. Other structures are the same as those of thefirst embodiment.

In the second embodiment, the crystallization suppressing layer 50suppresses crystallization of the first free layer 30 during theregularizing heat treatment process for the pinning layer 18. Therefore,as in the case of the first embodiment, the first free layer 30 can bemaintained in an amorphous state. In order to exchange-couple the firstfree layer 30 to the second free layer 32, it is preferable to set athickness of the crystallization suppressing layer 50 to 0.5 nm orthinner. The crystallization suppressing layer 50 may be thinned to oneatomic layer if the crystallization suppressing effect is ensured.

FIG. 6A shows the relation between a resistance change rate of thetunneling magnetoresistance device of the second embodiment and anapplied magnetic field. For the comparison sake, FIG. 6B shows therelation between a resistance change rate of a tunnelingmagnetoresistance device not provided with the crystallizationsuppressing layer 50 and an applied magnetic field. The maximumresistance change rate of the tunneling magnetoresistance device of thesecond embodiment was about 62%, whereas the maximum resistance changerate of the comparative example was about 17%. Coercive forces of thetunneling magnetoresistance devices of the second embodiment andcomparative example were 4.9 Os and 4.3 Os, respectively. It can be seenthat a large resistance change rate can be obtained by disposing thecrystallization suppressing layer 50.

It is possible to use as the material of the crystallization suppressingfilm 50, other conductive materials capable of suppressingcrystallization of the first free layer 30. Hf, Zr, Pd and the like maybe cited as usable material of the crystallization suppressing film 50.

FIG. 7 shows the main portion of the surface facing a magnetic recordingmedium, of a magnetic head including the tunneling magnetoresistancedevice of each of the first and second embodiments. An alumina film 76is formed on a base body 75 made of Al₂O₃—TiC or the like. A reproducingunit 80 is disposed on the alumina film 76, and an induction typerecording unit 90 is disposed on the reproducing unit 80.

The induction type recording unit 90 includes a lower magnetic pole 91,an upper magnetic pole 92 and a recording gap layer 93 disposed betweenthe poles. The upper magnetic pole 92 has a width corresponding to atrack width of the magnetic recording medium. The induction typerecording unit 90 further includes a yoke (not shown) for magneticallycoupling the lower magnetic pole 91 to the upper magnetic pole 92, and acoil (not shown) wound around the yoke. As a recording current flowsthrough the coil, a recording magnetic field is induced.

The lower magnetic pole 91 and upper magnetic pole 92 are made of softmagnetic material. Material having a large saturation magnetic fluxdensity, such as Ni₈₀Fe₂₀, CoZrNb, FeN, FeSiN, FeCo alloys may bepreferably used as the material of the lower magnetic pole 91 and uppermagnetic pole 92. The induction type recording unit 90 may be replacedby a recording unit having another structure.

Next, the structure of the reproducing unit 80 will be described. Alower electrode 81 is formed on the alumina film 76. A tunnelingmagnetoresistance device 85 is formed on a partial surface area of thelower electrode 81. The tunneling magnetoresistance device 85 has thesame structure as that of the tunneling magnetoresistance device of thefirst or second embodiment.

An insulating film 82 covers the sidewall of the tunnelingmagnetoresistance device 85 and the surface of the lower electrode 81continuous with the sidewall. Magnetic domain control films 83 aredisposed on both sides of the tunneling magnetoresistance device 85.Each of the magnetic domain control films 83 has a lamination structureof, e.g., a Cr film and a ferromagnetic CoCrPt film stacked in thisorder from the lower electrode 81 side. The magnetic domain controlfilms 83 make each of the pinned layers and free layers constituting thetunneling magnetoresistance device 85 have a single magnetic domain tothereby prevent generation of Barkhausen noises.

An alumina film 86 is formed on the tunneling magnetoresistance device85 and magnetic domain control films 83, and an upper electrode 87 isformed on the alumina film 86. A portion of the upper electrode 87penetrates the alumina film 86 and is electrically connected to theupper surface of the tunneling magnetoresistance device 85.

The lower electrode 81 and upper electrode 87 are made of soft magneticalloy such as NiFe and CoFe, and has a function as a magnetic shieldingfunction as well as a sense current flow path. A conductive film of Cu,Ta, Ti or the like may be disposed at the interface between the lowerelectrode 81 and tunneling magnetoresistance device 85.

The reproducing unit 80 and induction type recording unit 90 are coveredwith an alumina film, a carbon hydride film or the like in order toprevent corrosion and the like.

A sense current flows through the tunneling magnetoresistance device 85in a thickness direction thereof. A change in the tunnel resistance ofthe tunneling magnetoresistance device 85 is detected as a voltagechange.

FIG. 8A is a cross sectional view of a magnetic random access memory(MRAM) using the tunneling magnetoresistance device of the first orsecond embodiments, and FIG. 8B is an equivalent circuit of MRAM.Disposed on the surface of a silicon substrate 60 are a reproducing wordline 62, a MOS transistor 63, a recording word line 68, a bit line 69and a tunneling magnetoresistance device 70. The reproducing word line62 and recording word line 68 are in one-to-one correspondence andextend in a first direction (a direction perpendicular to the drawingsheet surface of FIG. 8A, a vertical direction in FIG. 8B). The bit line69 extends in a second direction (horizontal directions in FIGS. 8A and8B) crossing the first direction.

The MOS transistor 63 is disposed at a cross point between thereproducing word line 62 and bit line 69. The reproducing word line 62serves also as the gate electrode of the MOS transistor 63. Namely, theconduction state of the MOS transistor 63 is controlled by a voltageapplied to the reproducing word line 62.

The tunneling magnetoresistance device 70 is disposed at a cross pointbetween the recording word line 68 and bit line 69, and has the samestructure as that of the tunneling magnetoresistance device of the firstor second embodiment.

As a recording current flows through the recording word line 68 and bitline 69, a magnetization direction changes in the free layer of thetunneling magnetoresistance device 70 positioned at the cross point ofthe recording word line 68 and bit line 69. Data is written by changingthe magnetization direction. In tunneling magnetoresistance devicesdisposed at positions different from the cross point between therecording word line 68 and bit line 69 through which the recordingcurrent flowed, data is not written because a magnetic field is notgenerated having an intensity sufficient for changing the magnetizationdirection of the free layer.

The lowermost conductive layer of the tunneling magnetoresistance device70 is connected to one impurity diffusion region 61 of the MOStransistor 63 via a wiring 67 and a plurality of plugs 64 penetrating amultilayer wiring layer and isolated wirings 65. The uppermostconductive layer of the tunneling magnetoresistance device 70 isconnected to the bit line 69. Namely, the wiring 67 and bit line 69 areused as the electrodes for applying a sense current through thetunneling magnetoresistance device 70 in the thickness directionthereof.

The other impurity diffusion region 61 of the MOS transistor 63 isconnected to a plate line 66 via a plug 64. As the MOS transistor 63 ismade on-state, current depending on the resistance of the tunnelingmagnetoresistance device 70 flows between the bit line 69 and plate line66. By judging a magnitude of this current, data can be read.

By utilizing the same structure as that of the first or secondembodiment for the tunneling magnetoresistance device 70, it is possibleto lower the coercive force of the free layer and increase a currentchange amount. It is therefore possible to lower the recording currentand retain a large margin when recorded data is reproduced.

The present invention has been described in connection with thepreferred embodiments. The invention is not limited only to the aboveembodiments. It will be apparent to those skilled in the art that othervarious modifications, improvements, combinations, and the like can bemade.

1. A tunneling magnetoresistance device comprising: a pinned layer madeof ferromagnetic material having a fixed magnetization direction; abarrier layer disposed over the pinned layer and having a thicknessallowing electrons to transmit therethrough by a tunneling phenomenon; afirst free layer disposed over the barrier layer and made of amorphousor fine crystalline soft magnetic material which changes a magnetizationdirection under an external magnetic field; and a second free layerdisposed over the first free layer and made of crystalline soft magneticmaterial which changes a magnetization direction under an externalmagnetic field and being exchange-coupled to the first free layer. 2.The tunneling magnetoresistance device according to claim 1, wherein thefirst free layer is made of the soft magnetic material of CoFe addedwith at least one element selected from a group consisting of B, C, Al,Si and Zr.
 3. The tunneling magnetoresistance device according to claim1, wherein the first free layer is made of CoFeB and a B concentrationis 10 atom % or higher.
 4. The tunneling magnetoresistance deviceaccording to claim 1, wherein the second free layer is polycrystallinehaving a face centered cubic structure, and has non-orientation or has a(111) plane oriented preferentially in parallel to a substrate surface.5. The tunneling magnetoresistance device according to claim 1, whereina coercive force of the second free layer is smaller than a coerciveforce of the first free layer.
 6. The tunneling magnetoresistance deviceaccording to claim 1, further comprising a crystallization suppressinglayer disposed between the first and second free layers, thecrystallization suppressing layer preventing the first free layer frombeing crystallized by inheriting a crystal structure of the second freelayer.
 7. The tunneling magnetoresistance device according to claim 6,wherein the crystallization suppressing layer is made of Ta.
 8. A methodfor manufacturing a tunneling magnetoresistance device, comprising stepsof: (a) forming a pinning layer made of antiferromagnetic material on asupport substrate; (b) forming a pinned layer over the pinning layer,the pinned layer being made of ferromagnetic material whosemagnetization direction is fixed by an exchange interaction with thepinning layer; (c) forming a barrier layer over the pinned layer, thebarrier layer having a thickness allowing electrons to transmittherethrough by a tunneling phenomenon; (d) forming a first free layermade of amorphous or fine crystalline soft magnetic material over thebarrier layer; (e) exposing a surface of the first free layer tonitrogen plasma; (f) forming a second free layer made of crystallinesoft magnetic material over the first free layer exposed to the nitrogenplasma; and (g) conducting a regularizing heat treatment process for thepinning layer by disposing a lamination structural body between thesupport substrate and the second free layer in a magnetic field.
 9. Themethod for manufacturing the tunneling magnetoresistance deviceaccording to claim 8, wherein the step (g) is performed under acondition that crystallization will not progress from an interfacebetween the first and second free layers toward an inside of the firstfree layer.
 10. The method for manufacturing the tunnelingmagnetoresistance device according to claim 8, wherein the first freelayer is made of the soft magnetic material of CoFe added with at leastone element selected from a group consisting of B, C, Al, Si and Zr. 11.The method for manufacturing the tunneling magnetoresistance deviceaccording to claim 8, wherein the first free layer is made of CoFeB anda B concentration is 10 atom % or higher.
 12. The method formanufacturing the tunneling magnetoresistance device according to claim8, wherein the second free layer is polycrystalline having a facecentered cubic structure, and has non-orientation or has a (111) planeoriented preferentially in parallel to a surface of the supportsubstrate.
 13. The method for manufacturing the tunnelingmagnetoresistance device according to claim 8, wherein a coercive forceof the second free layer is smaller than a coercive force of the firstfree layer.
 14. A method for manufacturing a tunneling magnetoresistancedevice, comprising steps of: (a) forming a pinning layer made ofantiferromagnetic material on a support substrate; (b) forming a pinnedlayer over the pinning layer, the pinned layer being made offerromagnetic material whose magnetization direction is fixed by anexchange interaction with the pinning layer; (c) forming a barrier layerover the pinned layer, the barrier layer having a thickness allowingelectrons to transmit therethrough by a tunneling phenomenon; (d)forming a first free layer made of amorphous or fine crystalline softmagnetic material over the barrier layer; (e) forming a crystallizationsuppressing layer over the first free layer; (f) forming a second freelayer made of crystalline soft magnetic material over thecrystallization suppressing layer; and (g) conducting a regularizingheat treatment process for the pinning layer by disposing a laminationstructural body between the support substrate and the second free layerin a magnetic field, wherein the crystallization suppressing layersuppressing the first free layer from being crystallized by inheriting acrystal structure of the second free layer during the step (g).
 15. Themethod for manufacturing the tunneling magnetoresistance deviceaccording to claim 14, wherein the first free layer is made of the softmagnetic material of CoFe added with at least one element selected froma group consisting of B, C, Al, Si and Zr.
 16. The method formanufacturing the tunneling magnetoresistance device according to claim14, wherein the first free layer is made of CoFeB and a B concentrationis 10 atom % or higher.
 17. The method for manufacturing the tunnelingmagnetoresistance device according to claim 14, wherein the second freelayer is polycrystalline having a face centered cubic structure, and hasnon-orientation or has a (111) plane oriented preferentially in parallelto a substrate surface.
 18. The method for manufacturing the tunnelingmagnetoresistance device according to claim 14, wherein a coercive forceof the second free layer is smaller than a coercive force of the firstfree layer.
 19. A magnetic head comprising: a pinned layer made offerromagnetic material having a fixed magnetization direction; a barrierlayer disposed over the pinned layer and having a thickness allowingelectrons to transmit therethrough by a tunneling phenomenon; a firstfree layer disposed over the barrier layer and made of amorphous or finecrystalline soft magnetic material which changes a magnetizationdirection under an external magnetic field; and a second free layerdisposed over the first free layer and made of crystalline soft magneticmaterial which changes a magnetization direction under an externalmagnetic field and being exchange-coupled to the first free layer.
 20. Amagnetic memory comprising: a tunneling magnetoresistance device;recording means for applying a magnetic field to the tunnelingmagnetoresistance device to change magnetization directions of first andsecond free layers of the tunneling magnetoresistance device; andreproducing means for applying a sense current through the tunnelingmagnetoresistance device to detect a resistance of the tunnelingmagnetoresistance device, wherein the tunneling magnetoresistance devicecomprises: a pinned layer made of ferromagnetic material having a fixedmagnetization direction; a barrier layer disposed over the pinned layerand having a thickness allowing electrons to transmit therethrough by atunneling phenomenon; the first free layer disposed over the barrierlayer and made of amorphous or fine crystalline soft magnetic materialwhich changes a magnetization direction under an external magneticfield; and the second free layer disposed over the first free layer andmade of crystalline soft magnetic material which changes a magnetizationdirection under an external magnetic field and being exchange-coupled tothe first free layer.